1. Field of the Invention
The present invention relates to a photographic silver halide emulsion, to a silver halide photosensitive material, and to a photothermographic material. More particularly, the invention relates to an ultrathin tabular photographic silver halide emulsion having an average silver iodide content of 40 mol % or higher, to a silver halide photosensitive material, and to a photothermographic material.
2. Description of the Related Art
(Tabular Silver Iodide Grain)
Photosensitive silver halide emulsions for use in photographic technologies generally contain silver chloride, silver bromide, or a silver salt comprising a combination of chloride ion and bromide ion, to each of which a small amount of iodide is added.
Photosensitive silver iodide emulsions are rarely used in photographic technologies, but they are known in this technical field. Silver halide emulsions using grains containing silver iodide of different phases have been reported. Phosphoric acid silver iodide photographic emulsion resulting from coprecipitation of silver with iodide and phosphate has also been reported. However, there have been no reports on different silver iodide phases.
The crystal structure of silver iodide has been studied by crystallographers, especially those involved in photography. It is widely known that silver iodide can exist in three different crystal forms. A silver halide crystal in the most stable form is hexagonal wurtzite-type silver iodide, and is called β phase silver iodide. In addition, silver iodide in a face-centered cubic form, called γ phase silver iodide, is also stable at room temperature. Silver iodide in a body-centered cubic structure, which is stable only at temperatures of about 147° C. or higher and is called α phase silver iodide, also exists.
Regarding the shape of silver iodide grains, many shapes have been reported and tabular silver iodide crystals have been observed. With regard to the method for preparing tabular silver iodine crystals, a preparation method using an excess amount of iodine ions which yields a hexagonal crystal structure composed mainly of β phase silver iodide has been reported.
Concerning tabular silver iodide having a high aspect ratio, a method for preparing tabular silver iodide having a face-centered crystal structure, a thickness of less than 0.3 μm, and an average aspect ratio of 8 or more has been reported as disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 59-119344 and 59-119350. As a particularly desirable method, disclosed is a method in which the pAg is kept in a range from 1.0 to 2.0 during precipitation of silver iodide and the temperature in the reaction vessel is kept in a range of from 30° C. to 50° C.
A photosensitive silver halide is required to have grains in a uniform average grain size and uniform shape. To realize stable photographic properties and express the photographic characteristic of gradation as required according to applications, grains with these uniform distributions have generally been prepared and, when necessary, mixed or layered.
(Photothermographic Material)
In recent years, in the medical field and the graphic arts field, there has been a strong desire for a dry photographic process from the viewpoints of environmental conservation and economy of space. Further, the development of digitization in these fields has resulted in the rapid development of systems in which image information is captured and stored in a computer, and then when necessary processed and outputed by communicating it to a desired location where the image information is outputed onto a photosensitive material using a laser image setter or a laser imager, and developed to form an image at the location on the photosensitive material. It is necessary for the photosensitive material to be able to record an image with high-intensity laser exposure and that a clear black-tone image with a high resolution and sharpness can be formed.
While various kinds of hard copy systems using a pigment or a dye, such as ink-jet printers or electrophotographic systems, have been distributed as general image forming systems using such digital imaging recording material, images in the digital imaging recording material obtained by such a general image forming system are insufficient in terms of image quality (sharpness, granularity, gradation, tone) needed for medical images used in making diagnoses and high recording speed (sensitivity). These kinds of digital imaging recording materials have not reached a level at which they can replace medical silver halide film processed with conventional wet development.
A thermographic system using an organic silver salt has already been known. This system has an image forming layer including a reducible silver salt (for example, an organic silver salt), a photosensitive silver halide, and if necessary, a toner for controlling the color tone of silver, dispersed in a binder.
A photothermographic material forms a black silver image by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed on the exposed region. There is much literature in which photothermographic materials are described, and the Fuji Medical Dry Imager FM-DP L is an example in practice of a medical image forming system using a photothermographic material that has been marketed.
Since this kind of image forming system utilizing an organic silver salt has no fixing step, undeveloped silver halide remains inside the film after thermal development. Thus, there have intrinsically been two serious problems in the system.
One problem is that of instability in preserving an image after a thermal developing process, particularly fogging due to print-out when the material is exposed to light. As a way to improve the print-out, a method making use of silver iodide is known. However, the sensitivity of silver iodide grains known until now is extremely low, and silver iodide grains do not achieve a level of sensitivity that can be used in an actual system. And when a measure for preventing recombination between photoelectrons and positive holes is effected to improve the sensitivity, there is an inherent problem that the characteristic of having good print-out resistance will be lost.
As a way of increasing the sensitivity of a silver iodide photographic emulsion, academic literature discloses, for example, using a halogen acceptor such as sodium nitrite, pyrogallol, or hydroquinone, or immersion in an aqueous silver nitrate solution, or sulfur sensitization at a pAg of 7.5, and the like. However, the sensitization effect of these halogen acceptors is very small and extremely insufficient for use in photothermographic materials of the invention.
Another problem is that light scattering due to the remaining silver halide grains may cause cloudiness whereby the film turns translucent or opaque and image quality is degraded. To solve this problem, ways in which the grain size of photosensitive silver halide grains is made fine (to within a range of practical use of 0.08 μm to 0.15 μm) and the addition amount is reduced as much as possible to suppress the cloudiness caused by the silver halide have been practically employed. However, the compromise results in decreasing the sensitivity further, the problem of cloudiness is neither completely solved, and a dark milky color continues to remain and generate haze in the film.
In the case of a conventional wet developing process, the remaining silver halide is removed by processing with a fixing solution containing a silver halide solvent after the developing process. For the silver halide solvent, many kinds of inorganic and organic compounds are known which can form complexes with silver ions.
Even in the case of a dry thermal developing process, many attempts to introduce similar fixing measures in the material have been made. For example, a method has been proposed where a compound capable of forming complexes with silver ions is incorporated in the film and the silver halide is solubilized (usually referred to as fixing) through thermal development. However, this proposal only applies to silver bromide and silver chlorobromide, and the process also requires an additional heat treatment step for fixing, and the heating conditions require a high temperature within a range of 155° C. to 160° C. Thus, the system is one in which fixing is difficult to achieve.
In another proposal, a separate sheet (referred to as a fixing sheet) that includes a compound able to form complexes with silver ions is prepared, and after thermally developing the photothermographic material to form an image, the fixing sheet is overlaid on the developed photothermographic material, heating is carried out and the remaining silver halide is dissolved and removed. However, since this proposal requires two sheets, from a practical viewpoint the obstacles are that the processing step is complicated and the operational stability of the process is hard to maintain, and that there is a necessity to discard the fixing sheets after processing, resulting in generation of waste.
As another fixing method usable in thermal development, a method is proposed where a fixing agent for the silver halide is encapsulated in microcapsules, and thermal development releases the fixing agent and causes it to act. However, it is difficult to achieve a design that effectively releases the fixing agent. A method for fixing using a fixing solution after thermal development is also proposed, but it requires a wet process and therefore is not adequate for a completely dry process.
As described above, known methods for improving the turbidity of film have negative effects, and there have been substantial difficulties in their practical application.
Attempts have also been made at applying the above-mentioned photothermographic material as photosensitive material for photographing. The “photosensitive material for photographing” as used herein means a photosensitive material on which images are recorded by a one-shot exposure through a lense, rather than by writing the image information by a scanning exposure with a laser beam or the like. Conventionally, photosensitive materials for photographing are generally known in the field of wet developing photosensitive materials, and include films for medical use such as direct or indirect radiography films and mammography films, various kinds of photomechanical films used in printing, industrial recording films, films for photographing with general-purpose cameras, and the like.
For example, an X-ray photothermographic material coated on both sides containing tabular silver iodobromide grains using a blue fluorescent intensifying screen is described in JP-A No. 59-142539. As another example, a photosensitive material for medical use containing tabular grains that have a high content of silver chloride and have (100) major faces, and that are coated on both sides of a support, is described in JP-A No. 10-282606. Double-sided coated photothermographic materials are also disclosed in other patent documents. However, according to these known examples, although fine particle silver halide grains having a grain size of 0.1 μm or less do not cause further hazing, the sensitivity is very low. These grains are therefore not usable for practical applications in photographing. And conversely, when using silver halide grains having a grain size of 0.3 μm or more, because the remaining silver halide increases the degree of haze and adversely affects the print-out, there is severe deterioration of the image quality, and the grains are not usable for practical applications.
Photosensitive materials containing tabular silver iodide grains serving as the silver halide grains are well known in the wet developing field, but there have been no examples of applications thereof in a photothermographic material. The reasons are that, as mentioned above, the sensitivity is low, there is no effective sensitization method, and the technical barriers are even higher in thermal development.
To be usable for this kind of photosensitive material for photographing, the photothermographic material needs a higher degree of sensitivity as well as an even higher level of image quality with respect, for example, to the degree of haze of the obtained image.